One commercial process for manufacturing 1,4-butanediol involves isomerization of propylene oxide to allyl alcohol, rhodium-catalyzed hydroformylation of allyl alcohol to 4-hydroxybutanal, and catalytic hydrogenation of 4-hydroxybutanal to give 1,4-butanediol. Hydroformylation of allyl alcohol gives, in addition to 4-hydroxybutanal, a substantial proportion of 3-hydroxy-2-methylpropanal. The aldehyde products are extracted into water, and are hydrogenated in aqueous media using a Raney nickel catalyst to give 1,4-butanediol and 2-methyl-1,3-propanediol.
Raney nickel, a powdered catalyst, is useful for a slurry-type hydrogenation process, but not for a fixed-bed process. A goal in the field is to develop aqueous hydrogenation catalysts that are useful for a fixed-bed process. A large capital savings is potentially available in switching from a conventional slurry process to a fixed-bed process, and this potential savings provides a substantial incentive to develop viable catalyst systems useful for a fixed-bed process. Unfortunately, most fixed-bed supports are acidic, and tend to promote undesirable dehydration of 3-hydroxy-2-methylpropanal. Fixed-bed catalyst systems that give high selectivity to 2-methyl-1,3-propanediol are needed.
U.S. Pat. Nos. 4,083,882 (Taylor et al.) and 4,263,449 (Saito et al.) teach hydrogenation of aqueous 4-hydroxybutanal/3-hydroxy-2-methylpropanal mixtures using slurry-phase Raney nickel catalysts, but do not teach to use aqueous hydrogenation catalysts useful for a fixed-bed process.
Relatively acidic aqueous hydrogenation catalysts tend to catalyze unwanted cyclodehydration reactions. For example, when 4-hydroxybutanal is hydrogenated to give 1,4-butanediol, some of the 1,4-butanediol can cyclodehydrate to give tetrahydrofuran. Tetrahydrofuran may, in fact, be the desired end product; however, the manufacturer would like to be able to control the process to produce exclusively either 1,4-butanediol or tetrahydrofuran. Thus, hydrogenation catalyst systems that can minimize the amount of tetrahydrofuran produced in a 1,4-butanediol process are desirable.
U.S. Pat. No. 4,933,473 (Nimomiya et al.) teaches a process for producing neopentyl glycol by hydrogenating hydroxypivaldehyde in aqueous media in the presence of a platinum--ruthenium--tungsten (Pt--Ru--W) catalyst system. The reference teaches that the Pt--Ru--W catalyst has a higher activity and longer service life than catalysts containing Pt and Ru, either singly or in combination (column 2, lines 64-68). The reference does not address the problem of dehydration of 3-hydroxy-2-methylpropanal during hydrogenation of 4-hydroxybutanal/3-hydroxy-2-methylpropanal mixtures. In fact, because hydroxypivaldehyde lacks a hydrogen .beta.- to the hydroxyl group, dehydration cannot occur with hydroxypivaldehyde.
An improved process for hydrogenation of aqueous hydroxyaldehydes is needed. Preferably, the catalyst used in the process would have good activity and little tendency to promote dehydration of 3-hydroxy-2-methylpropanal or cyclodehydration of 1,4-butanediol so that yields of 2-methyl-1,3-propanediol would be high, and formation of isobutyl alcohol and tetrahydrofuran would be minimized. A process that could use a fixed-bed catalyst would save capital expenses and simplify product separation.